1. Technical Field
The present invention generally relates to wireless communication systems and in particular to identification of neighbor cells in wireless communication systems.
2. Description of the Related Art
Many frequency division duplex (FDD) networks deployments implementing the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (E-UTRA) standard can be synchronous and all time division duplex (TDD) deployments are expected to be synchronous. In a synchronous network, the Evolved Universal Terrestrial Radio Access Network Node Bs (eNodeBs) are aligned in symbol timing and radio-frame timing within a tolerance of the order of few microseconds (e.g., TS 36.133 requirement is +/−3 us for TDD operation). The primary synchronization channel (P-SCH or PSCH) and secondary synchronization channel (S-SCH or SSCH), which are transmitted by neighboring eNodeBs in subframes #0 and #5, can interfere with each other at a user equipment (UE) receiver for FDD (and similarly subframes pairs (#0, #1) and (#5, #6) for TDD). TS 36.211 defines 3 PSCH sequences and 168 SSCH sequences for each PSCH sequence to provide a total of 3*168=504 physical cell identities (PCID).
Network deployments where two neighbor eNodeBs on the same carrier end up with the same PSCH sequence is expected to be common since there are only three PSCH sequences. In principle, in one or two consecutive subframes over which PSCH and SSCH are transmitted, the propagation channel can be assumed to remain substantially unchanged between the two channels, and the PSCH may be used to obtain the channel reference for processing SSCH. However, in a synchronous deployment, for example, the serving cell signal strength may be strong enough to prevent the UE from being able to detect a neighbor/interferer cell with the same PSCH sequence as the serving cell. This is because, when the serving cell signal strength dominates at the UE receiver, if the UE tries to use PSCH sequence to estimate the channel reference, the UE ends up estimating the propagation channel from the serving cell rather than estimating the propagation channel from the neighbor cell. This (corrupted) channel estimate often leads to failure in identifying a newly detectable neighbor cell.